This invention relates generally to transistor base drive circuits and more specifically to a transistor base drive circuit for rapidly and efficiently switching a high current bipolar load transistor.
The advent of high current transistors capable of switching hundreds of amperes at high voltages has led to the development of improved switching mode power conditioners such as inverters, battery chargers, chopper circuits and the like. By fabricating switching mode power conditioners of such high current transistors in place of conventional four layer or pnpn semiconductor switching devices such as thyristors, switching mode power conditioner efficiency can be improved and power conditioner cost and size can be reduced.
To obtain good switching mode power conditioner performance, it is necessary that each power conditioner transistor turn-off quickly, that is to say, that each transistor be rendered rapidly conductive and nonconductive, respectively. This requires that the transistor base drive circuit closely regulate transistor base drive current. Ideally, a base drive circuit should supply the transistor with a large, fast rising pulse of base current to rapidly render the transistor conductive. Once the transistor is rendered conductive and becomes saturated, sufficient but not excessive base drive current should be supplied to maintain transistor saturation thus necessitating that the transistor base drive circuit supply the transistor with base drive current proportional to transistor collector current. To effect rapid substantial nonconductivity of the transistor once transistor turn-off has been initiated, the base drive circuit should reverse bias the transistor base with respect to the transistor emitter.
Heretofore, various base drive circuits have been proposed to effect rapid transistor switching. A common prior-art proportional base drive circuit comprises a transformer having a primary winding and first and second secondary windings. Typically, the first secondary winding of the transformer is coupled across the base-emitter junction of a bipolar load transistor and the second secondary winding is coupled in series with either the collector or the emitter of the load transistor. Initially, with the primary of the transformer energized such that the rate of change of flux in the primary is zero, no current is induced in either secondary winding, and thus the load transistor remains nonconductive. When the primary winding is deenergized, causing a change in magnetic flux, current is induced in the first secondary winding, causing the load transistor to become conductive and current to pass through the second secondary winding of the transformer. The current in the second secondary winding causes an opposite polarity current to be induced in the first secondary transformer winding, thus causing transformer base current to be proportional to transistor load current, that is, the current through the collector-to-emitter portion of the transistor. When the transformer primary winding is re-energized, a current opposite in direction to load transistor base drive current is induced in the first secondary transformer winding, causing load transistor base drive current to decrease to zero, thereby rendering the transistor nonconductive.
While such prior art base drive circuits maintain transistor saturation, once the transistor saturation is reached, by supplying the transistor with base drive current proportional to transistor load current, such prior art base drive circuits do not achieve rapid load transistor turn-off. This is because, after the transformer secondary winding is de-energized, the transformer secondary winding maintains decreasing conduction of load transistor base current, causing the load transistor to briefly remain conductive even after de-energization of the transformer primary.
In contrast, the present invention concerns an improved proportional base drive circuit for rapidly switching a high current bipolar load transistor.